|Ask the Experts|
July 11, 2017
BGA Rework - with or without solder paste
During a BGA Remove and Replace process, is it preferable to use a mini stencil and re-deposit solder paste on the site? Or is the fact that the pads are already tinned enough to produce a quality reflow?
|Expert Panel Responses|
There is an advantage to applying a fresh solder paste print, after BGA removal on a PCB. Afterall, this better replicates the original assembly process on the SMT line. With a Lead Free Solder, it is well documented that the wetting properties are quite different compared to a Tin/Lead Solder. Because of this "non-wet" issue, the flux within the newly applied solder will certainly help the new BGA solder to all pads more efficiently. One could argue that simply applying flux would be enough. And I would agree, it is possible to only use flux, but with a large area BGA and an unconfirmed volume of solder left on the PCB, there is better chance of success with solder printed on the PCB. If using Tin/Lead, the same problem of the ball wetting to the pad stays consistent. But Tin/Lead solder is a bit more forgiving, both from an alignment and wetting prospective. Finetech has worked with a number of customers, particularly in the automotive industry, where they have chosen not to apply new solder to a rework process. In some cases, reworking in nitrogen will also improve this process. Customers will rely only on the existing solder left on the PCB with a fresh deposit of flux. Thousands and thousands of boards have been reworked with this method with no integrity issues that I am aware of. It is important to conduct a design of experiments with both scenarios. Depending on the type of product, the volume (or lack thereof) could cause problems with humidity or hot/cold temp cycle testing. Customers that Finetech has worked with have conducted these same experiments and have proven that the solder joint integrity is maintained. A common challenge with a densely populated assembly is actually getting the solder onto the pads. There are various mini-stencil solutions on the market, and Finetech has created a hands-free, machine based Direct Component Printing module. Originally designed for QFN/MLF rework, with careful stencil design, customers are using this module to print solder paste onto the BGA balls of the new component. The rework system picks up the component, aligns, and places the BGA.
In most cases BGA rework can be achieved, and is often more reliable, without addition of solder paste (i.e., using only the solder balls). This assumes a good site prep and proper flux process. In cases where there may be planarity issues, solder paste can be beneficial. This is often seen with metal lid BGA and BGA connectors/sockets. When adding solder past to a BGA device, we have found it much easier and more relaible to add the past to the component rather than the board. Most rework system suppliers offer fixtures to perform this process.
VJ Technologies, Inc.
My input is for Tin Lead applications flux should work OK for tinned pads, but for Lead Free you want to put paste on the PCB pad or paste on the BGA ball. I have seen recent interest in placing paste on the BGA ball itself, which eliminates the problem of tightly packed components next to the BGA package when using a mini stencil for pad paste printing.
Vice President Technology
In my opinion, it must be best to try and replicate the original SMT process during rework. Therefore, one would tend to head towards using paste during rework, although many people do not. However, with lead-free applications, where producing a good soldered joint is more difficult, adding solder paste will help. The main functions of solder paste - during BGA rework - are to provide the flux necessary for reflow, to assist with creating good wetted joints and also to take out any complanarity issues. Unfortunately, it is difficult to apply paste to the pcb due to lack of space around the BGA site. Therefore, many people now use a stencil based system to apply paste to the balls of the BGA (CPF - component print frame). The problem with CPF is there may now be a 'lack of paste issue' (it is difficult to get sufficient paste onto the balls of a BGA)...which means there will be a lack of flux available...so this is then countered by applying a small amount of liquid flux to the pcb in addition to the paste on the BGA.
Although you may be able to re-attach a new BGA and have it pass electrical tests, you may be causing reliability issues later for two reaons.
Director of Application Engineering
The best process is to remove the component, remove the old solder from the board using desoldering braid and a variable power, fixed temperature soldering system fitted with a blade tip. This will ensure the site is flat. Then clean the old flux from the PCB with an approved solvent. Now if you are using solder paste, you need to apply the solder paste directly to the solder balls on the component using a solder paste plate. If you prefer paste flux, you need to apply the flux with a flux dip plate to ensure a uniform and repeatable application. Place the component and reflow the part with a convection rework machine. These steps will ensure a high quality and reliable reworked PCB.
Regional Sales Manager
OK International Inc.
I have to say for any BGA application the simplest way is to use dip paste designed for POP technology. It works like a dream and I just don't know why all of the suppliers do not push the product as a rework product. Even with its higher price it makes life so much easier.Bob Willis, bobwillisonline.com, UK
As several others have pointed out above, utilizing a rework process that most closely replicates the original manufacturing process is always preferred. While stenciling paste on the prepared BGA site may seem simple, it is without question an operation that requires a high level of operator skill to successfully master. Often times on highly complex boards, the BGA site may surrounded closely by numerous components, making it challenging to fit and secure a mini metal stencil flat to the PCB.
Even once the stencil is secured, operator technique is required to hand squeegee a clean print of paste through the stencil and onto the pads without any unwanted smearing that could cause bridges. Flexible, adhesive backed BGA rework stencils are available that help to reduce these challenges.
Circuit Technology Center
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